In a medium bowl, whisk together flour, sugar, baking powder and salt. Cut in cubes of butter with a pastry blender or two knives until mixture resembles coarse crumbs.

In a small bowl, whisk together milk, egg and vanilla. Pour into the dry ingredients and mix just until combined. Fold in chopped thin mints and chocolate chips. Evenly divide the batter between the wells of a 12 cup muffin tin lightly coated with nonstick spray.

To prepare the topping

In a medium bowl, stir together chopped thin mints, flour, sugar and salt. Mix in softened butter until crumbly. Evenly sprinkle the crumbly mixture on top of the muffin batter.

Place into the oven and bake until the tops spring back when lightly pressed in the center or a toothpick inserted into the center comes out mostly clean with a few moist crumbs attached, about 20 to 25 minutes. Remove from the oven and place pan on a wire rack for 5 minutes before carefully turning the muffins out to cool completely.